Backflow preventing circuit capable of preventing reverse current efficiently

ABSTRACT

This patent specification describes a backflow prevention circuit which includes a first switch configured to conduct or to shut down a current path from an input terminal to an output terminal, a logic circuit configured to binarize an input voltage at the input terminal based on an output voltage at the output terminal and to output a binary signal and a shutdown circuit configured to cause the first switch to shut down independently of a switching control signal in accordance with the binary signal output from the logic circuit. The switching control signal performs a switching control of the first switch. The logic circuit outputs a shutdown signal to shut down independently of the switching control signal when the input voltage becomes smaller than the output voltage.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a Rule 1.53(b) continuation of application Ser. No.11/636,308, filed Dec. 7, 2006, now U.S. Pat. No. 7,423,471 the entirecontents of which are incorporated by reference herein.

TECHNICAL FIELD

The present disclosure relates to a backflow prevention circuit, andmore particularly to a backflow prevention circuit capable of preventingreverse current efficiently.

BACKGROUND

Recently, a variety of electric equipments such as a mobile phone, apersonal computer, an electric home appliance, and so on, has beenwidely developed. It has been studied to make such electric equipmentmore compact, to have higher functional capability and lower powerconsumption. The electric equipment commonly includes a switchingcircuit between a power supply and a load to have a margin of safety.

Further, the electric equipment may include more complex circuit whichmay need a multiple power supplying system in order for the electricequipment to have a high performance. The multiple power supplyingsystem supplies multiple voltages and a variety of power capacities.Namely, it has become more important to control the multiple powersupplying system by switching each power supply so as to obtain a safeequipment having a low power consumption, and avoid faulty operation.

Thus, a switching circuit is generally employed to protect electricequipment from damages due to overheat and protect a power supply, suchas battery, etc., from a reverse current flowing from a load to thepower supply. It is desirable for the switching circuit to have lesspower consumption, because the switching circuit is an auxiliary circuitfor the electric equipment. Such a switching circuit is called abackflow prevention circuit.

FIG. 1 illustrates an example of a conventional backflow preventioncircuit 100. The backflow prevention circuit 100 includes a MOS (metaloxide semiconductor) switch 103, a diode 109 and a gate drive circuit110. The diode 109 and the MOS switch 103 are connected in seriesbetween a power supply 107 and a load 108.

When a voltage of the power supply 107 at an input terminal IN is largerthan a voltage of a cathode of the diode 109 at an output terminal OUT,a current flows through the diode 109 because a forward voltage isapplied on the diode 109. Meanwhile, when the voltage of the cathode ofthe diode 109 at the output terminal OUT is larger than the voltage ofpower supply 107 at the input terminal IN, the current through the diode109 is restricted and may be a small reverse current which is defined bya current-to-voltage characteristic of the diode 109.

In the backflow prevention circuit 100 of FIG. 1, however, there is apower loss at the diode 109 due to the forward current of the diode 109.Further, a resistance between the power supply 107 and the load 108 isincreased due to a resistance of the diode 109 in addition to aresistance of the MOS switch 103.

FIG. 2 illustrates another example of the conventional backflowprevention circuit 200. The backflow prevention circuit 200 includes aMOS switch 113, a comparator 111, a bias voltage source 112 and a gatedrive circuit 120. The bias voltage source 112 is employed to improve anoise margin of the backflow prevention circuit 200 and is connectedbetween the power supply 107 and an inverted-input terminal of thecomparator 111. The MOS switch 113 is formed of a P-MOS (p-channel metaloxide semiconductor) transistor. A voltage of the bias voltage source112 is set to be smaller than the voltage of the power supply 107.

The comparator 111 compares voltages between an output voltage at theoutput terminal OUT and the voltage of the power supply 107 at the inputterminal IN. When the output voltage becomes lower than a predeterminedvoltage Vc, the comparator 111 outputs a high level signal to a gate ofthe P-MOS transistor (MOS switch 113) so as to fix the MOS switch 113 tobe off. The predetermined voltage Vc is defined by the followingformula:Vc=Vp−Vbwhere Vp is the voltage of the power supply 107 and Vb is the biasvoltage source 112. Thus, a reverse current is prevented by setting theMOS switch 113 to be in shutdown state. The MOS switch 113 is off in ashutdown state.

In the backflow prevention circuit 200 of FIG. 2, however, minimumvoltage for the operation is relatively high because the comparator 111may be formed of a differential amplifier which requires a largervoltage from the power supply to operate. Further, there may be apenalty in power consumption because the differential amplifier mayinclude a constant current source which generates current constantly.Moreover, an operational voltage range may be narrower in comparison toother backflow prevention circuits, because an input voltage for thecomparator 111 is restricted to be within a narrower voltage range.

Furthermore, a full voltage range from the power supply voltage toground voltage may be needed to be input to the comparator 111 of FIG.2. Therefore, it may be necessary to employ so called Rail-to-Rail inputcircuit at an input part of the differential amplifier. The so calledRail-to-Rail outputs almost full voltage range from the power supplyvoltage to ground to the differential amplifier.

However, such input circuit may require approximately twice more circuitelements in comparison to conventional input circuit of the differentialamplifier. As a result, the backflow prevention circuit 200 may be alarge circuit in size and may have a larger power consumption incomparison with other backflow prevention circuits.

SUMMARY

This patent specification describes a novel backflow prevention circuitwhich, in a preferred embodiment, includes a first switch configured toconduct or to shut down a current path from an input terminal to anoutput terminal, a logic circuit configured to binarize an input voltageat the input terminal based on an output voltage at the output terminal,and output a binary signal, and a shutdown circuit configured to causethe first switch to shut down independently of a switching controlsignal in accordance with the binary signal output from the logiccircuit. The switching control signal is used to perform switchingcontrol of the first switch. The logic circuit outputs a shutdown signalto shut down independently of the switching control signal when theinput voltage becomes smaller than the output voltage.

According to another exemplary embodiment of this patent specification,the novel backflow prevention circuit further includes a pull-downresistor configured to form a pull-down connection so as to increase amargin of the backflow prevention circuit.

In another exemplary embodiment of this patent specification, the novelbackflow prevention circuit further includes a second switch configuredto perform switching in accordance with the binary signal of the logiccircuit, a current-limit resistor connected between a substrate gate ofthe first switch and the output terminal, and a third switch connectedbetween the substrate gate of the first switch and ground and configuredto perform switching at equal timing to the switching of the secondswitch in accordance with the binary signal of the logic circuit. Thefirst switch can be formed of N-channel MOS transistor and the shutdowncircuit is connected between a gate of the N-channel MOS transistor andground.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the disclosure and many of the attendantadvantages thereof will be readily obtained as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings, wherein:

FIG. 1 illustrates a conventional backflow prevention circuit;

FIG. 2 illustrates another conventional backflow prevention circuit;

FIG. 3 illustrates a backflow prevention circuit according to anexemplary embodiment of the present disclosure;

FIG. 4 illustrates a gate drive circuit of the backflow preventioncircuit of FIG. 3;

FIG. 5 illustrates an example of a backflow prevention circuit accordingto a second exemplary embodiment of the present disclosure;

FIG. 6 illustrates another example of a backflow prevention circuitaccording to the second exemplary embodiment of the present disclosure;and

FIG. 7 illustrates a backflow prevention circuit according to a thirdexemplary embodiment of the present disclosure.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In describing preferred embodiments illustrated in the drawings,specific terminology is employed for the sake of clarity. However, thedisclosure of this patent specification is not intended to be limited tothe specific terminology so selected and it is to be understood thateach specific element includes all technical equivalents that operate ina similar manner. Referring now to the drawings, wherein like referencenumerals designate identical or corresponding parts throughout theseveral views, particularly to FIG. 3, a backflow prevention circuitaccording to exemplary embodiments are described.

FIG. 3 illustrates a backflow prevention circuit 1 according to a firstexemplary embodiment of the present disclosure. The backflow preventioncircuit 1 is a switching circuit and connects a DC power supply 10 to aload 11. The DC power supply 10 is wired to an input terminal IN and theload 11 is wired to an output terminal OUT of the backflow preventioncircuit 1.

The backflow prevention circuit 1 includes a MOS switch 2, a gate drivecircuit 3 and a backflow prevention control circuit 4. The MOS switch 2is formed of N-MOS (n-channel metal oxide semiconductor) transistor. Thegate drive circuit 3 supplies a driving voltage to make the MOS switch 2on and off. The backflow prevention control circuit 4 is to prevent abackflow current from flowing back from the load 11 to the DC powersupply 10. The backflow prevention control circuit 4 is formed of theN-MOS transistor 7 and an inverter 8.

The MOS switch 2 performs as a first switch. The N-MOS transistor 7performs as a second switch and forms a shutdown circuit. The inverter 8forms a logic circuit. The backflow prevention control circuit 4 is acontrol circuit to prevent the backflow current from flowing back fromthe output terminal OUT to the input terminal IN, when the outputvoltage becomes larger than the input voltage.

The MOS switch 2 is connected between the input terminal IN and theoutput terminal OUT. A substrate gate of the MOS switch 2 is wired tothe output terminal OUT. The N-MOS transistor 7 is connected between agate of the MOS switch 2 and ground.

As for the inverter 8, an input terminal of the inverter 8 is wired tothe input terminal IN and an output terminal of the inverter 8 is wiredto a gate of the N-MOS transistor 7. A power to the inverter 8 issupplied from the output terminal OUT. A substrate gate of the N-MOStransistor 7 is wired to ground. The inverter 8 may be formed of avariety of combinations such as a P-MOS transistor and a resistor, aN-MOS transistor and a resistor, NMOS and PMOS transistors, etc.

The gate drive circuit 3 may be formed of a charge pump circuit as shownin FIG. 4. Referring to FIG. 4, the gate drive circuit 3 includes apower source 20, an oscillator 21, diodes 22 to 24, invertors 25 and 26and capacitors 27 to 29. The power source 20 supplies a predeterminedvoltage. The oscillator 21 generates a predetermined rectangular wave.

A plus terminal of the power source 20 is wired to an anode of the diode22 and a terminal of the capacitor 27. Another terminal of the capacitor27 is wired to an output terminal of the oscillator 21 and an inputterminal of the inverter 25. A cathode of the diode 22 is wired to ananode of the diode 23 and a terminal of the capacitor 28. An outputterminal of the inverter 25 is wired to another terminal of thecapacitor 28 and an input terminal of the inverter 26.

Similarly, a cathode of the diode 23 is wired to an anode of the diode24 and a terminal of the capacitor 29. An output terminal of theinverter 26 is wired to another terminal of the capacitor 29. A cathodeof the diode 24 is an output terminal of the gate drive circuit 3 and iswired to the gate of the MOS switch 2.

With this circuit configuration, when the output voltage becomes largerthan the input voltage, the inverter 8 outputs a high level signal tothe gate of the N-MOS transistor 7 so as to make the N-MOS transistor 7to be on. When the N-MOS transistor 7 is on, the gate of the MOS switch2 is grounded so that the MOS switch 2 is to be off. Thus, the backflowprevention circuit 1 is set to be in shutdown state independently of theoperation of the gate drive circuit 3. Thus, the backflow preventioncontrol circuit 4 controls to prevent the backflow current from flowingback from the output terminal OUT to the input terminal IN.

When the MOS switch 2 is stopped to operate, a reverse current throughthe N-MOS transistor 7 is relatively very small. Furthermore, even whilethe MOS switch 2 is being operated, only a very small leakage current isflowing from the gate drive circuit 3. Since the gate drive circuit 3 isformed of, for example, the charge pump circuit of FIG. 4, there may beno output current of the gate drive circuit 3 except a very smallleakage current to the gate of the MOS switch 2.

By the backflow prevention circuit 1 according to the first exemplaryembodiment, a power saving is achieved without additional power loss toa power loss due to the current flow from the input terminal IN to theoutput terminal OUT at a forward bias. Further, the backflow preventioncontrol circuit 4 is simple and is just formed of one inverter and oneN-MOS transistor. Therefore, it is easily integrated into an IC(integrated circuit) and a necessary area for the backflow preventioncontrol circuit 4 may be relatively small in the IC.

FIG. 5 illustrates a backflow prevention circuit 50 according to thesecond exemplary embodiment of the present disclosure. In a secondexemplary embodiment, a depression MOS transistor is introduced to thebackflow prevention control circuit 1 of FIG. 3. Namely, only differencefrom the first exemplary embodiment is that the depression NMOStransistor 31 is introduced.

Similar to the first exemplary embodiment, the backflow preventioncircuit 50 includes the MOS switch 2, the gate drive circuit 3 and abackflow prevention control circuit 4 a. The MOS switch 2 is formed ofN-MOS transistor. The gate drive circuit 3 supplies a gate voltage tocause the MOS switch 2 on and off.

The backflow prevention control circuit 4 a is a control circuit toprevent a backflow current from flowing back from the load 11 to the DCpower supply 10. The backflow prevention control circuit 4 a is formedof the N-MOS transistor 7, an inverter 8 and the depression NMOStransistor 31.

The depression NMOS transistor 31 is connected between the outputterminal of the inverter 8 and ground. A gate and a substrate gate ofthe depression NMOS transistor 31 is wired to ground to work as aconstant current source. The depression MOS transistor works as apull-down resistance to fix the output voltage of the inverter 8 to beground voltage.

The depression NMOS transistor 31 improves a noise margin of thebackflow prevention circuit 50 so as to avoid a following falseoperation.

When the output voltage is dropped slightly, the supply power voltage ofthe inverter 8 is also decreased because the power is supplied to theinverter 8 from the output terminal. By the decrease of the supply powervoltage of the inverter 8, an input voltage range, which allows theinverter 8 to output a low level signal in response to a predeterminedinput voltage range, is changed and becomes narrower.

The inverter 8 may output an almost equal voltage to the output voltageof the output terminal OUT to the gate of the N-MOS transistor 7, inaccordance with a slight drop of the input voltage at the input terminalIN. As a result, the N-MOS transistor 7 may be turned on.

By the backflow prevention circuit 50 according to the secondembodiment, however, it is possible to avoid an occurrence of suchfaulty operation by fixing the output voltage of the inverter 8 to beground voltage.

FIG. 6 illustrates another example of a backflow prevention circuitaccording to the second exemplary embodiment of the present disclosure.The backflow prevention circuit 60 of FIG. 6 is different from thebackflow prevention circuit 50 of FIG. 5 only in that a gate of thedepression NMOS transistor 31 is wired to the input terminal IN. Thebackflow prevention circuit 60 can obtain a similar effect as thebackflow prevention circuit 50 of the FIG. 5.

Thus, the depression NMOS transistor 31 is employed and is connectedbetween the output terminal of the inverter 8 and ground to improve thenoise margin of the backflow prevention circuits 50 and 60 in the secondexemplary embodiment.

In a switching circuits using MOS transistor switch, a substrate biaseffect of MOS transistor is to be considered. The substrate bias effectmay increase the threshold voltage of the MOS switch and affect theswitching performance of the MOS transistor switch.

In the switch circuit in which a MOS switch is provided between inputand output terminals and a substrate gate of the MOS switch is wired toground, when high voltage is input to the input terminal IN and the MOSswitch is to be on, the threshold voltage of the MOS switch is increaseddue to the substrate bias effect of MOS transistor. When the thresholdvoltage of the MOS switch is increased, a resistance at on-state of theMOS switch is increased.

In the first and second exemplary embodiments, the substrate gate of theMOS switch 2 is wired to the output terminal to avoid the substrate biaseffect.

However, there may be a possibility of an occurrence of a forward biasat a reverse current condition in which the output voltage is largerthan the input voltage. If the output voltage is higher than a voltageat the substrate gate of the MOS switch 2 at the reverse currentcondition, a large current may flow due to the forward bias which may beformed between the output terminal OUT and the substrate gate of the MOSswitch 2.

In a third exemplary embodiment, a N-MOS transistor 35 and a resistor 36are added to the backflow prevention circuit of FIG. 5 to avoid suchforward bias condition by applying ground voltage to the substrate gateof the MOS switch 2 at the reverse current condition.

FIG. 7 illustrates a backflow prevention circuit 70 according to thethird exemplary embodiment of the present disclosure. The backflowprevention circuit 70 includes the MOS switch 2, the gate drive circuit3 and a backflow prevention control circuit 4 b. The MOS switch 2 isformed of N-MOS transistor. The gate drive circuit 3 supplies a gatevoltage to cause the MOS switch 2 on and off.

The backflow prevention control circuit 4 b is a control circuit toprevent a backflow current from flowing back from the load 11 to the DCpower supply 10. The backflow prevention control circuit 4 b is formedof N-MOS transistors 7 and 35, the inverter 8, the depression NMOStransistor 31 and a resistor 36.

The N-MOS transistor 35 and the resistor 36 are connected in seriesbetween the output terminal OUT and ground. The connecting node of theN-MOS transistor 35 and the resistor 36 is wired to the substrate gateof the switch 2. The N-MOS transistor 35 forms a third switch. As forthe N-MOS transistor 35, a gate of the N-MOS transistor 35 is wired tothe output terminal of the inverter 8. The substrate gate of the N-MOStransistor 35 is wired to ground.

With this circuit configuration, when the output voltage becomes largerthan the input voltage, the inverter 8 outputs a high level signal tothe gate of the N-MOS transistor 7. The N-MOS transistors 7 and 35 areto be on at same time. Then, the substrate gate of the MOS switch 2 isgrounded. As a result, a backflow current is avoided by setting the MOSswitch 2 to be off and in shutdown state.

Thus, in the backflow prevention circuit 70 according to the thirdexemplary embodiment, the N-MOS transistor 35 is connected between thesubstrate gate of the MOS switch 2 and ground. The substrate gate of theMOS switch 2 is connected to the output terminal through the resistor36.

It is possible to avoid the reverse current by setting the MOS switch 2to be off and in shutdown state. Further, it is possible to avoid theoccurrence of a leakage current of the MOS switch 2 due to the forwardbias at the reverse current condition, in addition to obtaining similarbenefits as the second exemplary embodiment.

The resistor 36 limits a current through the N-MOS transistor 35. Apower consumption capacity of the resistor 36 is determined to allow apredetermined current from the output terminal OUT to ground at thereverse current condition.

The backflow prevention circuit 70 according to the third exemplaryembodiment is a modification of the backflow prevention circuit of FIG.5. However, the concept of this disclosure is also applicable to otherbackflow prevention circuits such as the backflow prevention circuits ofFIG. 3, FIG. 6 and etc.

Numerous additional modifications and variations are possible in lightof the above teachings. It is therefore to be understood that within thescope of the appended claims, the disclosure of this patentspecification may be practiced otherwise than as specifically describedherein.

This patent specification is based on Japanese patent application No.2005-355809 filed on Dec. 9, 2005 in the Japan Patent Office, the entirecontents of which are incorporated by reference herein.

1. A method of controlling a backflow prevention circuit, comprising thesteps of: conducting a current path from an input terminal to an outputterminal by turning on a first switch in accordance with a switchingcontrol signal; shutting down the current path from the input terminalto the output terminal by turning off the first switch in accordancewith the switching control signal; and binarizing an input voltage atthe input terminal based on an output voltage at the output terminaloutputting a binary signal, wherein the first switch is shut down inaccordance with the binary signal independently of the switching controlsignal when the input voltage becomes smaller than the output voltage.2. The method of claim 1, further comprising connecting a power supplyto a load through said backflow prevention circuit.
 3. The method ofclaim 2, further comprising connecting said power supply to said inputterminal and connecting said load to said output terminal.
 4. The methodof claim 1, further comprising preventing flow of a backflow currentfrom said output terminal to said input terminal, by operation of saidbackflow prevention circuit.
 5. A mobile phone including a backflowprevention circuit configured to prevent flow of a reverse current froma load In the mobile phone to a power supply of the mobile phone, saidbackflow prevention circuit comprising: an input terminal and an outputterminal a first switch configured to conduct or shut down a currentpath from said input terminal to said output terminal; a logic circuitconfigured to binarize an input voltage at the input terminal based onan output voltage at the output terminal, and output a binary signal;and a shutdown circuit configured to cause the first switch to shut downindependently of a switching control signal in accordance with thebinary signal output from the logic circuit the switching control signalbeing used for switching control of the first switch, wherein the logiccircuit outputs a shutdown signal to shut down the current pathindependently of the switching control signal when the input voltagebecomes smaller than the output voltage.
 6. The mobile phone of claim 5,wherein said backflow prevention circuit connects said power supply tosaid load.
 7. The mobile phone of claim 5, wherein said input terminalof said backflow prevention circuit is coupled to said power supply andsaid output terminal of said backflow prevention circuit is coupled tosaid load.
 8. The mobile phone of claim 5, wherein said backflowprevention circuit prevents flow of a backflow current from said outputterminal to said input terminal.
 9. Electronic equipment comprising: apower supply; a load consuming at least part of power supplied by saidpower supply; and a backflow prevention circuit configured to preventflow of a reverse current from said load to said power supply, saidbackflow prevention circuit comprising: a first switch configured toconduct or shut down a current path from an input terminal to an outputterminal; a logic circuit configured to binarize an input voltage at theinput terminal based on an output voltage at the output terminal, andoutput a binary signal; and a shutdown circuit configured to cause thefirst switch to shut down independently of a switching control signal inaccordance with the binary signal output from to logic circuit, theswitching control signal being used for switching control of to firstswitch, wherein the logic circuit outputs a shutdown signal to shut downthe current path independently of to switching control signal when toinput voltage becomes smaller than to output voltage.
 10. The electronicequipment of claim 9, wherein said backflow prevention circuit connectspaid power supply to said load.
 11. The electronic equipment of claim 9,wherein said input terminal is coupled to said power supply and saidoutput terminal is coupled to said load.
 12. The electronic equipment ofclaim 9, wherein said backflow prevention circuit prevents flow of abackflow current from said output terminal to said input terminal. 13.The electronic equipment of claim 9, wherein said electronic equipmentincludes a computer and said backflow prevention circuit is included insaid computer.